| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Research Reports |
PM Ludewig, PT, PhD, is Associate Professor, Program in Physical Therapy, Mayo Mail Code 388, The University of Minnesota, 420 Delaware St, Minneapolis, MN 55455 (USA)
Address all correspondence to Dr Ludewig at: ludew001{at}umn.edu
Lin and colleagues1 have undertaken an investigation to identify the resting or "loose-packed" position of the glenohumeral joint by quantifying the humeral head anterior and posterior translations and axial rotation range of motion across a variety of scapular-plane abduction positions in subjects who are healthy. I thank the authors for their efforts in contributing to the expansion of knowledge on shoulder biomechanics. Such studies provide a potential foundation for refinement of current diagnostic and treatment approaches for shoulder joint pathology in patients. From a measurement perspective, a specific strength of the study was the use of controlled forces and torques imposed on the joint during the test procedures.
Measurement of glenohumeral joint translation is particularly challenging in vivo. The authors used a technique to estimate the glenohumeral joint center that has been demonstrated to be stable in a plastic ball-and-socket model.2 However, any surface-based testing method is not immune from errors due to sensor and skin motion distinct from the underlying bone, particularly at higher angles of humeral elevation.3 Lin and colleagues present reliability data for their translation measures in the form of intraclass correlation coefficient (ICC) values. These values provide proportional variability information and suggest good within-day repeatability across trials for these methods. However, values are not presented distinctly for different abduction positions. Were ICC values consistent at all positions of glenohumeral abduction? Additional insight into the stability of the measures could be obtained by also reporting the standard error of measurement or root mean square variation between the trials. These reliability values then would be interpretable in the actual units of measurement (in this case, millimeters). Do the authors believe differential skin motion artifact across measurement positions may have confounded the differences in translation values reported in end-range abduction at all?
Lin and colleagues adopt a definition of the resting position as "the position of a joint in which the joint tissues are under the least amount of stress and where the joint capsule has its greatest laxity."1(pp 1669–1682) Measurements of the magnitude of anterior/posterior humeral head translations and axial rotations are assumed to be related to joint capsule laxity, as direct capsular tension or stress measures in vivo are not currently viable. Subsequently, the authors report 2 different rest positions based on average rotation and translation measurements across subjects. This interpretation raises a number of questions. First, is the use of average measurements to define the rest position the optimal approach with a small sample of 15 subjects of mixed sexes? The standard deviation values presented in Tables 3 and 4 are substantial. If classifying the position of maximal motion for each subject individually in one of the positions (0°=maximum), how many of these subjects would have a rest position at the 20-degree test positon, roughly equivalent to the average rest position? It would be interesting to see the distribution of positions where the greatest range of motion occurred across individual subjects.
The authors describe a single average glenohumeral joint abduction position as the position where the greatest translations occurred (24°) and a second average glenohumeral joint position where the rotational range of motion was greatest (50°).1 This single-position approach, however, versus reporting a range of positions, is not fully consistent with the statistical results. If I am following the figures correctly, in post hoc comparisons, no significant differences in translation values were reported between neutral and 50 degrees of glenohumeral abduction for total translation values. So is it appropriate to identify the translational rest position as 24 degrees, or rather as occurring within the range of positions from 0 to 50 degrees? Similarly, for total rotation values, no post hoc significant differences are reported between 30 and 50 degrees of glenohumeral abduction, so could not this range be defined as the range where the rest or loose-packed position occurred, rather than reporting the single average value of 50 degrees? If such an approach were used, could not a composite range of 30 to 50 degrees of glenohumeral abduction then contain the rest position for both total translation and total rotation values? Such an interpretation would seem consistent with the data reported and quite consistent with the reports in the literature cited by the authors of a range of 30 to 60 degrees for the glenohumeral positions of maximal motion.4,5 What was the minimal clinically important difference between test positions for translation and rotation values?
Finally, the concept of a "composite" measure of the rest position seems important to consider as an alternative to distinguishing translatory and rotational rest positions as distinct joint positions. Because the glenohumeral joint has limited bony congruency in any position, the rest or loose-packed position is predominately influenced by the joint capsule and other soft tissues. Different portions of the capsule are known to experience tension with different directions of motion testing. Subsequently, it is not surprising that the positions for maximal rotational and translational range of motion were not exactly the same if defined as single positions. As the authors point out, if superior/inferior and medial/lateral translatory measurements also had been completed, these motions might have demonstrated maximal mobility at other distinct glenohumeral joint positions. Is a series of unidirectional "rest" positions consistent with the originally intended definition of overall capsular laxity or capsular volume? Rather, do we need to develop a composite multidirectional measure of translatory and rotational range of motion to best capture the intended definition of overall capsular laxity inherent in the definition of the rest or loose-packed position? Using Lin and colleagues' average data from the present study,1 a 40-degree glenohumeral abduction position would appear to capture the combined maximal rotational and translatory motion measured. Values at this 40-degree abduction position for rotational and translatory motions are within a few degrees and a few millimeters of the peak values reported at the 2 distinct average rest positions.
I am also curious to know Lin and colleagues' thoughts regarding potentially applying this assessment of rest position to clinical populations and in clinical settings. Have the authors formulated hypotheses regarding how subgroups of clinical patients with various shoulder disorders might present clinically during the assessment of glenohumeral joint rest position? How specifically might results of such a clinical assessment be applied to the diagnostic process for subjects with shoulder pain? Finally, these techniques are currently performed clinically without controlled force and torque, and with manual stabilization of the scapula. Do the authors have suggestions for more quantitative measurement techniques to evaluate passive joint mobility within the constraints of a clinical setting?
As with any addition to the scientific literature, this article raises additional questions to consider in the pursuit of "translating" research findings toward improving the practice of patient care. I thank the authors for their contribution to the scientific literature and look forward to their responses.
 | References |
|---|
CiteULike 
Complore 
Connotea 
Del.icio.us 
Digg 
Reddit 
Technorati What's this?
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
